It has been recently considered that a particulate filter is incorporated in an exhaust pipe to collect particulates in exhaust gas, and a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen is arranged downstream of the particulate filter, urea water as a reducing agent being added at between the particulate filter and the selective reduction catalyst, thereby achieving simultaneous reduction of the particulates and NOx.
In such a case, the addition of the urea water to the selective reduction catalyst is at between the particulate filter and the selective reduction catalyst. Thus, in order to secure a sufficient reaction time for thermal decomposition of the added urea water into ammonia and carbon dioxide gas, a distance is to be lengthened from a urea-water adding position to the selective reduction catalyst. There is, however, a problem that the spaced-apart arrangement of the particulate filter and the selective reduction catalyst with a sufficient distance in between would significantly impair a mountability on a vehicle.
Accordingly, devised was an exhaust emission control device as shown in FIG. 1 comprising a particulate filter 3 for capture of particulates in exhaust gas 1 from an engine and a selective reduction catalyst 4 downstream of the particulate filter 3 and having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. The particulate filter 3 and the selective reduction catalyst 4 are encased by casings 5 and 6, respectively, and incorporated in parallel with each other in an exhaust pipe 2 through which the exhaust gas 1 from the engine flows; and an exit end of the particulate filter 3 is connected with an entry end of the selective reduction catalyst 4 through an S-shaped communication passage 7 so that the exhaust gas 1 discharged from the exit end of the particulate filter 3 is turned back in an opposite direction and is guided to the entry end of the adjacent selective reduction catalyst 4.
The communication passage 7 is the S-shaped structure comprising a gas gathering chamber 7A encompassing the exit end of the particulate filter 3 to gather the exhaust gas 1 just discharged from the exit end through substantially perpendicular turnabout, a mixing pipe 7B for extraction of the gathered exhaust gas 1 in the chamber 7A in a direction opposite to that of the exhaust gas flow in the particulate filter 3 and a gas dispersing chamber 7C for dispersion of the exhaust gas 1 from the mixing pipe 7B through substantially perpendicular turnabout and for encompassing the entry end of the selective reduction catalyst 4 to guide the dispersed exhaust gas 1 into the entry end of the catalyst. The entry end of the mixing pipe 7B is centrally provided with a urea water adding injector 8 for addition of the urea water into the mixing pipe 7B and directed to the exit end of the mixing pipe 7B.
In the example illustrated, an oxidation catalyst 9 is installed in the casing 5 before the particulate filter 3 to perform an oxidation treatment of unburned fuel in the exhaust gas 1. An ammonia reducing catalyst 10 is installed in the casing 6 after the selective reduction catalyst 4 to perform an oxidation treatment of surplus ammonia.
With such a configuration employed, the particulates in the exhaust gas 1 are captured by the particulate filter 3; and, downstream thereof, the urea water is added in the exhaust gas 1 midway of the mixing pipe 7B by the urea water adding injector 8 and is thermally decomposed into ammonia and carbon dioxide; then, on the selective reduction catalyst 4, NOx in the exhaust gas 1 is well reduced and purified by the ammonia. As a result, the simultaneous reduction of the particulates and NOx in the exhaust gas 1 are achieved.
In this case, the fact that the exhaust gas 1 discharged from the exit end of the particulate filter 3 is turned around in the opposite direction by the communication passage 7 and then guided to the entry end of the adjacent selective reduction catalyst 4 ensures a long distance from the added position of the urea water to the selective reduction catalyst 4 and thus ensures a sufficient reaction time for production of ammonia from the urea water.
Moreover, the fact that the particulate filter 3 and the selective reduction catalyst 4 are arranged in parallel with each other and the communication passage 7 is arranged between and alongside of the particulate filter 3 and selective reduction catalyst 4 makes an overall configuration compact in size, thereby substantially improving the mountability on the vehicle.
In the urea water adding injector 8 provided in the exhaust emission control device as mentioned in the above, urea crystals may precipitate in a nozzle of the injector under specific conditions such as sudden stop of the engine having been in operation with high exhaust temperature, which may bring about temporary malfunction of failed urea-water injection due to adhesion of the precipitating urea crystals.
Specifically, upon stop of the engine, a countermeasure is taken for prevention of the urea water from freezing, for example, during the stop of the engine in a cold district by purging the urea water to the urea water tank as soon as an ignition key is turned off (see, for example, undermentioned Patent Literature 1). As a result, at a next start of the engine, the injector 8 is to be idly operated for expulsion of the air in the passage followed by feed of the urea water.
When the nozzle of the injector 8 is clogged with the urea crystals in this case, air expulsion by the idle operation may fail into a situation that the urea water from the pump advancing while compressing the air does not arrive at the injector 8 so that the urea water cannot reach the nozzle of the injector 8 so as to dissolve the urea crystals.
In such situation, conventionally conducted are repeated operations such that the pump for feed of the urea water is temporarily stopped to reduce the pressure in the urea-water passage and the pump is restarted after lapse of a predetermined time to conduct pilot injection. Thus, the depressurization and pressurization are alternately repeated so as to resolve the clogging of the urea crystals and thus revive the injector 8.